PC enthusiasts sometimes talk that their “rails” are low and that they need a new power supply. First off, the “rails” that they are referring to are the 3.3V, 5V, and 12V lines that come off the power supply. The 3.3V and 5V generally power the digital devices like the motherboard, PCI slots, AGP slot, etc. The 12V is usually for motorized devices like the hard drives, CD-ROM, fans, etc. These lines are rated to run at a certain specification, usually somewhere around plus or minus 5%. Technically, if these lines stay within this margin, the system should run fine with no instabilities but if they do run out of specification a number of problems can arise. Usually power supplies go under the minimum voltage when used over a long period of time. When the rails are low this usually leads to blue screens and various other instabilities like random reboots. When these lines are overvolted problems arise again like drive failure, blue screens, damaged drives and damaged motherboards. Therefore, it is easy to see why it is important to have "clean" power - power that is stable, does not fluctuate, and within a tight tolerance.